1. Field of the Invention
The present invention relates to an image signal recording and reproducing system for recording an image signal on a recording medium and for reproducing a recorded image signal from the recording medium.
2. Description of the Related Art
Still video (SV) systems are conventionally known as one type of system for recording an image signal on a recording medium and for reproducing a recorded image signal from the recording medium. Such an SV system is arranged to form an image signal corresponding to a subject image by means of a solid-state image sensor such as a CCD-type image sensor or a MOS-type image sensor, to supply the resultant image signal to a signal processing circuit, and to record on a magnetic disk the image signal processed by the signal processing circuit.
FIG. 1 shows a schematic block diagram of the signal processing circuit of an SV system having a solid-state image sensor whose imaging surface is provided with R, G and B stripe filter elements.
Referring to FIG. 1, RGB signals are outputted from a solid-state image sensor 10 whose imaging surface is provided with R, G and B stripe filter elements, and the RGB signals are level-adjusted respectively by amplifiers 201, 202 and 203 so that the levels of the RGB signals become equal to one another when a white subject is imaged by the solid-state image sensor 10. Thereafter, the RGB signals are subjected to known gamma correction by a gamma correction circuit 204.
The RGB signals subjected to gamma correction are supplied to both a luminance signal forming circuit 208 and respective low-pass filters (LPF's) 205, 206 and 207. The luminance signal forming circuit 208 samples and holds each of the RGB signals and forms a wide-band luminance signal Y.sub.H through switching at predetermined timings. Subsequently, the wide-band luminance signal Y.sub.H outputted from the luminance signal forming circuit 208 is band-limited by a low-pass filter (LPF) 210 and outputted as a final luminance signal Y. In the meantime, the RGB signals supplied to the respective low-pass filters (LPF's) 205, 206 and 207 are band-limited and inputted to a matrix circuit 209. The matrix circuit 209 forms a narrow-band luminance signal Y.sub.L from the input RGB signals, and then forms and outputs two kinds of color-difference signals R-Y.sub.L and B-Y.sub.L.
The luminance signal Y thus formed is frequency-modulated, while the color-difference signals R-Y.sub.L and B-Y.sub.L are converted into a color-difference line-sequential signal and then frequency-modulated. Subsequently, the luminance signal Y and the color-difference line-sequential signal are frequency-multiplexed and recorded on a magnetic disk called a video floppy disk. During reproduction, the recorded signal is reproduced from the video floppy disk by a reproducing apparatus, and a restored image signal is displayed on a television (TV) monitor as a visual image. Accordingly, the resolution of images attained by the aforesaid SV system is limited by the resolution of images obtainable with an existing TV system such as an NTSC system.
To improve the resolution of images of the existing TV system, a number of novel television systems, such as a high-definition television (HDTV) system, have recently been put into practice. Such a novel TV system has approximately 1,000 scanning lines and a horizontal signal band which can accommodate such a large number of scanning lines. Accordingly, in the field of SV systems, it is desired to develop a system capable of recording and reproducing a high-resolution still image of the order of 1,000 pixels.times.1,000 pixels (per square image area extracted from an entire picture) obtainable with the HDTV system or the like.
To meet the above-described demand, it has been proposed to provide a compatible high-definition still video (CHSV) system which can record and reproduce a high-resolution still image signal while retaining compatibility with conventional SV systems.
Such a CHSV system will be described in brief below.
FIG. 2 is a partial schematic view showing an example of the construction of a color filter attached to the imaging surface of an image sensor for use in the CHSV system. The color filter is attached to an image sensor having pixels the number of which is about 1,300 pixels.times.about 1,000 pixels, and consists of Y-signal filter elements which are arranged in a checkered manner and R- and B-signal filter elements which are arranged in line-sequential order. The CHSV system is arranged to form a luminance signal and a color-difference line-sequential signal by outputting, as described below, signals stored in the pixels of the image sensor to which is attached a color filter such as that shown in FIG. 2. More specifically, luminance signals for one field are formed by outputting the signals stored in pixels which are arranged on every fourth line, such as lines A.sub.1, A.sub.2, . . . In this manner, luminance signals for four fields including A, B, C and D fields which correspond to lines A.sub.i, B.sub.i, C.sub.i, and D.sub.i (.sub.i : positive integer) shown in FIG. 2, are formed.
As is known, color-difference signals C.sub.R and C.sub.B (C.sub.R =R-Y, C.sub.B =B-Y) need to be converted into a color-difference line-sequential signal. For instance, after the color-difference signal C.sub.R has been formed from the signals stored in the pixels of the line A.sub.1 of FIG. 2, it is necessary to form the color-difference signal C.sub.B from the signals stored in the pixels of the line B.sub.1. Accordingly, in the CHSV system, the signals stored in the pixels of even lines A.sub.2n and B.sub.2n, C.sub.2n and D.sub.2n (n: integer) of each field are outputted in reverse order, respectively, to form a color-difference line-sequential signal.
After the thus-formed luminance signals and color-difference line-sequential signals for four fields are each frequency-modulated, these signals are multiplexed for each field. The multiplexed signals for four fields are respectively recorded on four successive tracks of a video floppy disk.
FIG. 3(a) is a schematic view showing a track recording pattern which is formed in a case where, after image signals for the A and B fields are simultaneously recorded through a two-channel head, the two-channel head is made to travel by two tracks to simultaneously record image signals for the C and D fields. FIG. 3(b) is a schematic view showing a track recording pattern which is formed in a case where a four-channel head is used to simultaneously record image signals for the A and B fields with a space for one track interposed therebetween and simultaneously record image signals for the C and D fields with the track on which the B field is recorded interposed therebetween.
If the two-channel head is used to effect recording, by reproducing the B and D tracks it is possible to reproduce a still image signal for one frame which conforms to a conventional TV system by means of a reproducing apparatus which conforms to a conventional SV format. If the four-channel head is used to effect recording, by reproducing the A and C tracks or the B and D tracks it is possible to reproduce a still image signal for one frame which conforms to a conventional TV system.
If a high-resolution still image signal is to be reproduced, the image signals for four fields recorded on the A to D tracks are fetched into a memory. The luminance signals and color-difference signals of the image signals are interpolated in the memory, and the resultant high-resolution still image signal is outputted from the memory.
The above-described type of conventional apparatus has a number of disadvantages. One disadvantage is that it is impossible to partially extract an arbitrary area from an image having a HDTV-grade resolution of the order of, for example, 1,000 vertical pixels, convert it into a video signal conforming to an existing TV system such as the NTSC system or a
system, and display the arbitrary area on a monitor or the like.
Another disadvantage of the conventional apparatus is that a user cannot determine whether an image signal reproduced by an image signal recording and reproducing system is an image signal reproduced by a conventional SV system, which can be displayed only on a monitor conforming to the existing TV system such as the NTSC or PAL system, or an image signal reproduced by the CHSV system, which can be displayed on both a monitor conforming to the existing TV system and a monitor conforming to the CHSV system. In the conventional apparatus having such a disadvantage, it is likely that an image signal reproduced by the CHSV system is displayed only on the monitor conforming to the existing TV system. If the user is to determine whether the reproduced image signal is an image signal reproduced by the CHSV system or the SV system, he himself must confirm whether it can be displayed on a CHSV-system monitor by supplying the signal thereto. For this purpose, the user must carry out extremely complicated operations.
The above-described CHSV system is capable of recording and reproducing a still image signal on the basis of not only a CHSV recording and reproducing mode for recording and reproducing a high-resolution still image signal by using four tracks in the above-described manner, but also a field recording and reproducing mode for recording a still image signal for one field on one track on the basis of a conventional SV format and for reproducing a still image for one picture conforming to a conventional TV system by using the still image signal for one field, or on the basis of a frame recording and reproducing mode for recording a still image signal for one frame over two tracks and for reproducing a still image for one picture conforming to the conventional TV system by using the still image signal for one frame. However, if a video floppy disk containing both a track having a still image signal recorded in a CHSV recording mode and a track having a still image signal recorded in a field or frame recording mode is to be reproduced, the following problems will be encountered: 1) if a high-image-quality monitor for displaying a high-resolution still image and a standard-image-quality monitor for a conventional TV system are not connected to the CHSV system, the still images recorded in these recording modes cannot be displayed; and 2) if both a high-image-quality monitor and a standard-image-quality monitor are connected to the CHSV system, a still image will be selectively displayed on either of the monitors in accordance with the kind of recording mode. This operation may be an extreme nuisance to a viewing person.
In addition, in the CHSV system, even if it is desired only to confirm the contents of the image recorded on each track of a video floppy disk, it takes a long time to output the contents to a monitor because the signal recorded in the CHSV recording mode is reproduced in a CHSV reproduction mode. Accordingly, the conventional arrangement does not fully utilize the advantage of the CHSV system--the ability to reproduce the signal recorded in the CHSV recording mode in the field or frame reproduction mode and display it on a standard-image-quality monitor.